The present invention relates to a method of fabricating a phase mask for processing optical fibers, and an optical fiber with a Bragg diffraction grating, which is manufactured using the optical fiber-processing phase mask. More particularly, the present invention relates to a method of fabricating a phase mask for making a diffraction grating in an optical fiber used for optical communications, etc. using ultraviolet laser light, and an optical fiber with a Bragg diffraction grating, which is manufactured using the mask.
Optical fibers have brought about breakthroughs in the globalization of communications to make high-quality and large-capacity inter-oceanic telecommunications feasible. So far, it has been known that a Bragg diffraction grating is provided in an optical fiber by creating a periodic refractive index profile in an optical fiber core along the optical fiber, and the magnitude of reflectivity and the width of the wavelength characteristics of the diffraction grating are determined by the period and length and the magnitude of refractive index modulation of the diffraction grating, whereby the diffraction grating can be used as a wavelength division multiplexer for optical communications, a narrow-band yet high-reflection mirror used for lasers or sensors, a wavelength selection filter for removing extra laser wavelengths in fiber amplifiers, etc.
However, the wavelength at which the attenuation of a silica optical fiber is minimized and which is suitable for long-distance communications is 1.55 xcexcm. It is thus required that the grating spacing be about 500 nm in order to allow the optical fiber diffraction grating to be used at this wavelength. At the beginning, it was considered difficult to make such a minute structure in an optical fiber core; that is, a Bragg diffraction grating was provided in the optical fiber core by a sophisticated process comprising a number of steps, e.g., side polishing, photoresist coating, holographic exposure, and reactive ion beam etching. For this reason, much fabrication time was needed, resulting in low yields.
In recent years, however, a method of fabricating a diffraction grating by irradiating an optical fiber with ultraviolet radiation to cause a refractive index change directly in an optical fiber core has been developed. This ultraviolet irradiation method has been steadily put to practical use with the advance of peripheral technologies, because of no need of any sophisticated processes.
Since the grating spacing is as fine as about 500 nm as mentioned above, this method using ultraviolet light is now carried out by an interference process using the interference of two light beams, a writing-per-point process wherein single pulses from an excimer laser are focused to make diffraction grating surfaces one by one, an irradiation process using a phase mask having a grating, etc.
Regarding the interference process using the interference of two light beams, a problem arises in conjunction with the quality of the beams in the lateral direction, i.e., spatial coherence. A problem with the writing-per-point process is on the other hand that strict step control of the submicron order is needed to focus light on a small point for writing light on many surfaces. Another problem arises in conjunction with processability.
To solve these problems, attention has focused on the irradiation process using a phase mask. According to this process, a phase mask 21 comprising a quartz substrate provided on one surface with grooves of given depth at a given pitch is irradiated with ultraviolet laser light (of 190 to 3.00 nm wavelength) 23 to give a refractive index. change to a core 22A of an optical fiber 22, thereby producing a grating (diffraction grating)., as shown in FIG. 5(a). For a better understanding of an interference pattern 24 Qn the core 22A, the pattern 24 is exaggerated in FIG. 5(a). FIG. 5(b) is a sectional view of the phase mask 21, and FIG. 5(c) is a top view corresponding to FIG. 5(b). The phase mask 21 has a binary phase type of diffraction grating structure where a substrate is provided on one surface with grooves 26 having a depth D at a repetition pitch P, with a strip 27 substantially equal in width to each groove being provided between adjacent grooves 26.
The depth of each groove 26 on the phase mask 21 (the difference in height between strip 27 and groove 26) D is chosen such that the phase of the ultraviolet laser light (beam) that is exposure light is modulated by xcfx80 radian. Thus, zero-order light (beam) 25A is reduced to 5% or less by the phase mask 21, and chief light leaving the mask 21 is divided into +first-order diffracted light 25B containing at least 35% of diffracted light and xe2x88x92 first-order diffracted light 25C, so that the optical fiber 22 is irradiated with the +first-order diffracted light 25B and xe2x88x92 first-order diffracted light 25C to produce an interference fringe at a given pitch, thereby providing a refractive index change at this pitch in the optical fiber 22.
The grating produced in the optical fiber using such a phase mask 21 as mentioned above has a constant pitch, and so the phase mask 21 used for grating production is provided with grooves 26 at a constant pitch.
Such a phase mask is produced by writing an electron beam on positions, corresponding to grooves 26, on the quartz substrate coated with an electron-beam resist, using an electron-beam writing system and etching out the written portions.
To achieve a narrow-band optical fiber diffraction grating, however, such a phase mask 21 as mentioned above is required to have a size of the order of 100 mm in the repetition direction of grooves 26 (in the sectional direction in FIG. 5). In addition, it is not easy to continuously expose a phase mask blank to writing beams in one operation. Thus, such an optical fiber diffraction grating is fabricated by writing the entire region of a phase mask blank with writing beams by a step-and-repeat process wherein the entire region of the phase mask blank is divided into small segments (at an interval of about 7 mm). One segment is first written with writing beams while a writing stage is fixed, and then the writing stage is moved by one segment to write the next segment with writing beams. This operation is repeated while the segments are connected to one another, so that the entire region of the phase mask blank can be sequentially written with the writing beams.
However, a problem with this step-and-repeat process is that there is a phase shift (a stitching error) in the repetition period of grooves 26 in the connection of adjacent segments to each other. In an optical fiber diffraction grating fabricated using a phase mask having such a stitching error, a number of unnecessary peaks other than essential side lobes occur on both sides of the center Bragg peak, as can be seen from the wavelength vs. reflectivity relation shown in FIG. 8.
In view of such problems with the prior art as mentioned above, one object of the present invention is to provide a method of fabricating an optical fiber-processing phase mask which can reduce or substantially eliminate stitching errors ascribable to a deterioration in the wavelength selectivity of the optical fiber diffraction grating to be fabricated. The present invention also includes an optical fiber with a Bragg diffraction grating, which is manufactured using such a phase mask for processing optical fibers.
According to one aspect of the present invention, this object is accomplished by the provision of a method of fabricating an optical fiber-processing phase mask comprising a transparent substrate provided on one surface with a grating form of repetitive groove-and-strip pattern, in which an optical fiber is irradiated with light diffracted by said repetitive pattern to produce an interference fringe by interference of diffracted light of different orders, thereby providing a diffraction grating in the optical fiber, wherein:
at an exposure step, a writing stage with a phase mask blank placed thereon is continuously fed in one direction while a portion of said phase mask blank corresponding to a groove or a strip perpendicular to said feeding direction is scanned with a writing beam, thereby continuously writing said beam on an entire region of said phase mask blank to be written.
According to another aspect of the present invention, there is provided a method of fabricating an optical fiber-processing phase mask comprising a transparent substrate provided on one surface with a grating form of repetitive groove-and-strip pattern, in which an optical fiber is irradiated with light diffracted by said repetitive pattern to produce an interference fringe by interference of diffracted light of different orders, thereby providing a diffraction grating in the optical fiber, wherein:
when, at an exposure step, a writing beam is written on an entire region of a phase mask blank to be written while segments smaller than said entire region to be written are sequentially scanned with said writing beam while said segments are connected to one another in a direction perpendicular to a groove or a strip, adjacent segments are allowed to overlap one another at a part of end areas thereof.
In the present invention, beam writing may be carried out using either an electron-beam writing system or a laser-light writing system.
In the present invention, the pitch of the grating form of repetitive groove-and-strip pattern is usually between 0.85 xcexcm and 1.25 xcexcm.
It is here noted that the difference in height between grooves and strips in the grating form of repetitive groove-and-strip pattern is preferably set such that the phase of optical fiber-processing ultraviolet radiation is shifted by approximately xcfx80 upon transmission.
The present invention also encompasses an optical fiber provided with a Bragg diffraction grating, which is produced using the optical fiber-processing phase mask fabricated by either one of the above two fabrication methods.
According to the present invention, the writing stage with the phase mask blank placed thereon is continuously fed in one direction at the exposure step while a portion of the phase mask blank corresponding to a groove or a strip perpendicular to the feeding direction is scanned with a writing beam, thereby continuously writing the entire region of the phase mask blank to be written. Alternatively, when, at the exposure step, a writing beam is written on the entire region of a phase mask blank to be written while segments smaller than said entire region to be written are sequentially scanned with said writing beam while said segments are connected to one another in a direction perpendicular to a groove or a strip, adjacent segments are allowed to overlap one another at a part of end areas thereof. Thus, there is no stitching error due to connections between the segments to be written, unlike the prior art. In the optical fiber provided with a Bragg diffraction grating which is produced using such a phase mask, no unnecessary peaks occur on both sides of the center Bragg peak.